First elected to the assembly as AM for South Wales West in 1999,[2] he was re-elected in 2003 and 2007 and served as the party's spokesman on economic development and transport for eight years. In the Third Assembly, he held the education and lifelong learning portfolio and also chaired the Assembly's Finance Committee. On 11 July 2007, he became the Shadow Minister for Education, Lifelong Learning and Skills in the National Assembly for Wales. As a member of the assembly's economic development and audit committees, he criticised the Welsh Government over a range of issues from transport infrastructure, European regional aid, public spending, and the Welsh economy. Cairns has also been a vocal opponent of the Scarweather Sands offshore wind farm development near Porthcawl.

In September 2008, Cairns admitted that he asked for a National Assembly rule to be "clarified" to determine whether he could claim expenses for a second home in Cardiff. A rule change introduced in late 2006 by the Assembly's House Committee allowed him to claim expenses related to a flat in Cardiff even though his main home was reclassified as being situated in the Vale of Glamorgan.[4]

While taking part in BBC Radio Cymru's weekly radio show, Dau o'r Bae, on 13 June 2008, Cairns was asked to apologise on air for referring to Italians as "greasy wops", and immediately did so.[5] He subsequently resigned from his post in the Shadow Cabinet on the following day.[6] He was re-appointed to the Shadow Cabinet as Shadow Minister for Local Government on 22 October 2008 after completion of a party investigation.[7]

Cairns was the Conservative Party's candidate for the Vale of Glamorgan at the 2005 general election, and was re-selected as Parliamentary candidate for the seat in July 2007. Cairns was suspended as the parliamentary candidate while the party carried out an investigation over his radio comments,[8] Cairns was reinstated as the parliamentary candidate on 22 October 2008.[7]

Cairns is a member of the ‘Curry Club’ group of Conservatives, a dining society set up in 2010 composed of Conservative MPs that were seen as independently-minded though not hostile to the then Prime Minister David Cameron.[9][10][11]

In 2011 Cairns became co-chairman of the newly formed All-Party Parliamentary Group on the Arch Cru Investment Scheme, intended to investigate the facts surrounding Arch Cru investment funds scandal and achieve justice for the victims.[12][13][14]

In January 2016, the Labour Party unsuccessfully proposed an amendment in Parliament that would have required private landlords to make their homes "fit for human habitation". According to Parliament's register of interests, Cairns was one of 72 Conservative MPs who voted against the amendment who personally derived an income from renting out property. The Conservative Government responded to the amendment by stating that they believed homes should be fit for human habitation but did not want to pass the new law that would explicitly require it.[16]

After re-election, in July 2017, Cairns announced that tolls on the Severn bridges from Wales to England would be abolished by the end of the following year,[19] a move which he said could boost the south Wales economy by about £100m a year.[20]

Cairns caused controversy in April 2018 when he announced the Severn Bridge between Wales and England was to be renamed 'Prince of Wales Bridge' to mark the 60th anniversary of Prince Charles becoming the Prince of Wales.[21][22][23][24] Cairns defended the decision by responding that a "silent majority" supported the name change, but a poll conducted by YouGov shortly after revealed 34% of respondents to be against the name change and only 17% in favour, while 47% had no strong feelings either way.[25][25]

Cairns has said he supports the proposed Swansea Bay Tidal Lagoon renewable energy power plant, although he has expressed reservations in regard to the financial viability of the project, stating in 2017 that "none of us would want it to happen if it’s not good value for money for the taxpayer."[26][27][28] Cairns came under criticism in 2018 when reports emerged the government was considering abandoning the project, Plaid Cymru politician Jonathan Edwards referring to Cairns as the "grim reaper of Welsh politics - the bearer of bad news" after Cairns highlighted concerns over the cost of the tidal lagoon relative to nuclear power stations.[29][30][31][32]

In June 2018, the UK government announced that the plans for the Swansea Bay Tidal Lagoon would be scrapped.[33] This led to Plaid Cymru tabling a motion of no confidence in Cairns at the Welsh Assembly, a vote that if passed would not have forced Cairns to resign as Welsh Secretary but rather would serve as symbolic disapproval.[34][35] The motion was defeated, with 9 in favour to 40 against, on 27 June.[36]

In Parliament, Cairns has previously served on the Public Administration Committee and Welsh Affairs Committee.[37]

Cairns believes the best route out of poverty is through employment. At a live BBC event in 2017, he said "Getting a job is absolutely the best way out of poverty", but that increasing the personal allowance for tax and raising the National Living Wage were also important.[38]

He has spoken in favour of fox hunting as he says it is a part of "countryside conservation".[39]

Cairns has claimed society is only on "step one" of gender equality, and further progress has to be made.[40][41] He believes all of society stands to gain from gender equality because the UK economy depends on "harnessing the talent of women, capitalising on the wealth of skill they bring to our workplaces".[42]

As part of the Downing Street Policy Board in 2013, Cairns asserted that it is the duty of the BBC to educate children about online dangers such as grooming and adult content, and implied it could be done through television shows such as EastEnders which he pointed out had educated the public in the past on issues like HIV, he said "the BBC's mission statement and stated public purposes fit perfectly for it to become the trusted source of advice on how to protect children online."[43][44] In relation to the television licence fee, Cairns described it as “probably the UK’s most regressive tax”, and stressed this meant transparency was vital.[45] Despite this, Cairns considers himself a "critical friend" of the BBC.[45]

Cairns supported remaining in the European Union in the 2016 UK European Union membership referendum. In the campaign period leading up to the referendum, in a government pamphlet sent to all homes across Wales, he stated: "One thing I think we can all agree on is that Europe needs reform. The Prime Minister has fought hard to get a deal which gives the UK the protections it needs." He further stated that the UK had achieved "a special status" within Europe and how the deal negotiated would allow Britain to avoid "the Euro, open borders or the prospect of ever-closer union."[46][47]

After the UK voted to leave the European Union, Cairns came out in support of Brexit. In 2018 he said he was "delighted" economic predictions about Brexit had been "proven wrong", adding: "I think people should be excited about the new opportunities as we leave the European Union."[48] He rejects the idea of the UK remaining in the EU single market in a 'soft' Brexit.[49] In November 2018 he endorsed Prime Minister Theresa May's negotiated agreement with the European Union on the United Kingdom's exit terms, though he said: "I don't like every element of this document because we've compromised".[50]

Cairns is a proponent of government decentralisation for Wales. He has noted specifically the example of English devolution and how it has created a "new dynamic" in England,[51] one which he would like Wales "to be able to respond to" by "empowering the regions, north Wales, west Wales” as part of its own "new dynamic."[52] Cairns envisions Wales as part of a potential "Western powerhouse", similar to the idea of the Northern Powerhouse in England, and in early 2018 set up the Severn Growth Summit to explore opportunities for boosting Welsh economic growth.[53][54][55]

Cairns lives in London and his Vale of Glamorgan constituency with his wife Emma and son Henri.[3][56]

He is an avid marathon runner, and as of 2018 has completed seven runs of the London Marathon. His London Marathon personal best time of 3:28:02 makes him the seventh fastest running MP of all time.[3][57][58] Through his marathon runs, he has raised money for NSPCC Wales and women's aid charity Atal y Fro.[3]